The invention relates to an encoder particularly for ultra fast high resolution flash analog-to-digital converters (ADC).
Two parts can be distinguished in every flash ADC: an analog and digital sections. The analog section of conventional N-bit flash ADCs consists of a reference source, chain of 2.sup.N - 1 resistors and the same number of comparators. The digital section consists of an encoder for sampling comparator output signals, correcting faulty codes and providing binary output code.
The digital output code from the analog section of a flash ADC provided by comparators is referred to as thermometer code since only one pair of adjacent bits is 0 and 1. The code must be sampled and stored as it is unstable due to changes of the ADC input signal, offset voltage drift of comparators, noise, etc.
An encoder is disclosed by Clark et al. in the abovementioned "Converter for Converting a High Frequency Video Signal to a Digital Signal" U.S. Pat. No. 4,211,999 dated Jul. 8, 1980. The thermometer code is stored in an input register which also provides a pair of section codes. Priority encoders convert the section codes into binary codes. One of the binary codes is selected by a multiplexer in response to a select bit provided from one of the priority encoders. The multiplexer provides the output code of the encoder. The priority encoders separately convert the section codes and binary codes are separately applied to the multiplexer. This results in a very complex structure at a high resolution of the flash ADC. Moreover, the employment of the priority encoders prior to the multiplexer results in large propagation delays, different for different binary codes. The input register is necessary. A very limited error correction is accomplished.
The huge number of the comparators in high resolution flash ADCs results in a very complex gate structure of the encoders. Certain kinds of errors caused by a false response of the comparators are not possible to correct thru a digital correction in the encoder, wherein a final error is large. For instance, the flash ADCs have a tendency to miss a code every few billion conversions, i.e. several times a minute when ADC is sampling at 100 MHz.
Recently invented conversion schemes result in significantly increased speed and resolution of the flash ADCs. Moreover, a possibility of anomalous response of the comparators is significantly reduced. A number of documents addresses these problems and in particular the abovementioned "High Resolution True Flash Analog-to-Digital Converter", "High Accuracy Reference Ladder", both filed on even date herewith and "Flash Analog-to-Digital Converter" U.S. Pat. No. 4,763,106 dated Aug. 9, 1988. All inventions are by the same inventor.